1. Field of the Invention
The present invention relates to optical fiber cross-connect switching. More particularly, it relates lo load balancing in Dense Wavelength Division Multiplexing optical cross-connect systems.
2. Description of the Related Art
Associated with the information revolution is a need to increase by many orders of magnitude the rate of information transfer. This can be accomplished with optical fibers and the method of Dense Wavelength Division Multiplexing (DWDM), in which many wavelength channels, each including a different narrow band of wavelengths of light and each carrying different information, are multiplexed onto a single optical fiber using an optical multiplexer. Optical signals carried on the various wavelength channels may be separated at the output of the optical fiber with an optical demultiplexer.
Optical fiber cross-connect switches may be used to direct the optical signals on some or all of the wavelength channels on a particular optical fiber to other optical fibers. Such optical fiber cross-connect switches include those described in U.S. Patent Application Ser. Nos. 09/999,878, 09/999,610, and 10/002,310, now issued U.S. Pat. No. 6,636,656, all of which are incorporated herein by reference in their entirety. Hence, optical signals on the various wavelength channels on an optical fiber may have originated at separate locations and traveled different distances in optical fiber. Since light is attenuated during transmission through optical fiber by an amount typically proportional to the distance traveled in optical fiber, the various wavelength channels on an optical fiber may carry different power levels.
Optical amplifiers such as Erbium Doped Fiber Amplifiers (EDFA) can amplify a wide wavelength band (spanning many wavelength channels), and thus compensate for transmission losses in optical fibers. If the power levels on the various wavelength channels carried by the optical fiber are not nearly equal at the input to the optical amplifier, however, the wavelength channel or channels of highest power may saturate the gain. Under such circumstances, the lower power wavelength channels might not be sufficiently amplified.
A variable optical attenuator is an optical device with which the amplitude or power level of an input optical signal may be attenuated by a variable amount to provide an output optical signal of a desired amplitude or power level. The power levels of the various wavelength channels on an optical fiber may be substantially equalized in a “load balancing” or “load equalization” process in which each wavelength channel is routed through a separate variable optical attenuator. Variable optical attenuators are described, for example, in U.S. Pat. Nos. 5,864,643 and 6,130,984. These devices require the insertion of additional hardware into an optical network. The additional hardware may be expensive, requires additional physical space, and may introduce unwanted attenuation of the optical signals.
It would be desirable to incorporate the function of a variable optical attenuator into an optical network without the insertion of additional optical elements.